More NorthUp Compass Information


Major Features
Specifications
Schematic and Pictorial
NorthUp Connections
General Description
Compass Virtues
Compass Options
Compass Calibration
IrDA Output
Antenna Lines
Additional Info


 

Major Features


  • Automatically provides azimuth-stabilized DF displays
  • DF display : 12 o'clock position = north ( north = "up" )
  • RS232 ( Agrello DF format ) input, RS232 and IrDA outputs
  • One-time calibration procedure, non-volatile EEPROM storage of data
  • CALIB trimpot provided to correct for centerline alignment errors
  • Top / bottom mounting selection strap
  • Spare DB15 pins for antenna control lines


    
    Specifications
    Dimensions : 0.9 x 2.4 inches, SMT components Inputs : Agrello DF RS232 message, 2400 baud Outputs : Agrello DF RS232 message, 2400 baud + IrDA LED output Resolution : 1 degree Repeatability : 2 degrees ( estimated ) Accuracy : 3 degrees ( estimated, good conditions ) Power : 7-14 VDC

    Schematic and Pictorial


    NorthUp [schematic diagram] ( GIF 76 Kb)
    NorthUp [board pictorial] ( GIF 47 Kb)


    NorthUp Connections



    General Description


    The NorthUp electronic compass is an integrated electronic compass that uses two orthogonal magneto-inductive sensors to detect vehicle heading. A microcomputer operates the compass circuits and performs measurements of the results. Software routines in the microcomputer convert the raw sensor data to vehicle heading data, expressed in degrees.

    The NorthUp compass accepts Agrello DF RS232 data from the DF, adds the ( measured ) vehicle heading to the DF bearing, and then re-transmits the DF data through a second RS232 port. As a result, the DF bearings produced by the NorthUp compass are expressed in magnetic degrees, ( 12 o’clock = magnetic north ) instead of relative degrees, ( 12 o’clock = vehicle heading ) and changes of vehicle heading no longer affect the DF bearing. An additional IrDa LED output is also provided, to drive a PalmOS display with the compass-corrected DF data.

    A simple calibration procedure must be performed ( once ) to correct for errors caused by magnetic masses located near the compass. Once performed, the data is stored in non-volatile EEPROM memory.


    Compass Virtues


    The NorthUp electronic compass automatically provides an azimuth-stabilized display for the PicoDopp DF. When employed, the 12 o’clock position of the DF display represents magnetic north, instead of the vehicle heading, and changes of vehicle heading do not affect the orientation of the DF display. This is particularly useful for the IBM and PalmOS display programs, since these programs ( visually ) integrate hundreds or even thousands of DF bearings, to provide a much more coherent display of DF data, ( bearing "history") especially for weak DF signals.

    Changes of vehicle heading will invalidate all the "history" data that was accumulated before the turn, since the "reference direction" ( for history data ) changes with vehicle heading. For example, if the DF is indicating a ( relative ) signal direction of 045 degrees, and the vehicle then executes a 90 degree turn to the left, the DF will then indicate a signal direction of 135 ( relative ) degrees, because of the turn. This is not a problem for a "realtime" display, but for a display that provides integration and display of DF "history", it is a problem.

    To deal with this issue, the display programs have manual inputs that allow the user to enter changes of vehicle heading, but this is a burdensome chore that requires faithful, vigilant attention on the part of the DF user…. every time the vehicle heading changes by more than a few degrees, the heading must be manually updated in the display program, to maintain validity of the DF data. ( that is, to prevent loss of validity for the DF "history" data )

    The NorthUp electronic compass completely "automates" this process… it liberates the DF operator from this chore, and allows the operator to pay attention to other matters that contribute more directly to the success of the hunt. Meanwhile, the NorthUp compass enables the display "integration routines" to be fully utilized, automatically.

    NOTE : The NorthUp compass cannot be used to "stabilize" the economy local displays, ( Economy Pelorus or Economy Numeric ) but it can be used to stabilize the MultiDisplay.


    Compass Options


    The standard NorthUp compass operates with Agrello format DF messages at 2400 baud. Other baud rates are available on request, but are not "field selectable". The NorthUp compass will operate with other DF units that generate the required RS232 Agrello DF format messages. Additional ( minor ) changes are negotiable, if the user desires some "special" feature not available in the standard NorthUp compass.

    The NorthUp compass cannot be used to stabilize the Economy Pelorus or Economy Numeric displays, but it can stabilize the MultiDisplay, by driving the MultiDisplay RS232 input from the NorthUp RS232 output. In fact, a switch can be added to select between the NorthUp and PicoDopp RS232 outputs, allowing selection of either a "relative bearing" display or a "north-stabilized" display.

    An alignment trimpot is provided to allow corrections for misalignment errors, between the compass centerline and the vehicle centerline. This trimpot has 500 degrees of adjustment range, ( 1.5 vehicle turns ) so the NorthUp compass may actually be installed with any orientation that is convenient. If access to the trimpot is ( for any reason ) inconvenient, the display programs also have a feature that allows manual entry of bearing corrections, to achieve the same effect.

    A strap is provided on the NorthUp compass to allow inverted mounting of the compass. When the strap is installed, the compass assumes it is oriented with the component side of the PC board facing "up". With this strap removed, the compass may be mounted upside down, so the component side faces down.


    Compass Calibration


    The NorthUp electronic compass has provisions to correct for compass errors caused by nearby magnetic masses. These errors are commonly called deviation errors. The NorthUp correction method is very effective, even for extreme errors caused by large magnetic masses located very close to the compass sensors.

    The correction method involves a simple procedure which is performed only once by the user, after the compass is installed in its final location on the vehicle. The resulting data is stored in non-volatile EEPROM memory in the NorthUp compass, and does not require further updates unless the magnetic environment of the compass ( for some reason ) changes.

    The procedure involves closing a user-provided CALIBRATION switch that connects to the NorthUp DB15 connector. While the switch is closed, the compass ( and vehicle ) is rotated through a 360 degree turn. During this time, the NorthUp compass accumulates data about the magnetic environment.

    When the 360 degree vehicle turn is completed, the CALIBRATION switch is opened, and the NorthUp compass calculates the necessary correction factors, saves them in EEPROM memory, and restarts the NorthUp computer, using the new correction factors.

    The 360 degree calibration turn may be executed at any reasonable speed, since the NorthUp compass generates about 10 bearings per second. There is no "time limit" on the duration of the turn.


    IrDA Output


    The NorthUp compass also generates an IrDA ( infrared ) data output, suitable for driving a PalmOS PDA display. The PicoDopp DF offers this output as a standard feature, for use with the ( free ) OptoDopp PDA display program, available on this website.

    PDA displays offer an economical yet sophisticated handheld DF display, with the additional virtue of cordless operation, using data sent from an infrared LED. For those who employ this feature of the PicoDopp DF, the IrDA output of the NorthUp compass allows the PDA to operate with the added benefit of a north-stabilized display.

    If desired, a switch can be added to select either the NorthUp IrDA output or the PicoDopp IrDA output, allowing selection of either a "relative bearing" display or a "north-stabilized" display.


    Antenna Lines


    As a convenience for the user, 4 pins on the DB15 connector are reserved for use as antenna control lines. The PicoDopp DF uses 4 lines to control antenna switching, and since the compass will ( very often ) be mounted near the antenna, these extra pins allow the user to fabricate a single control cable for all DF connections to the antenna. ( the RF coaxial cable must still run separately to the FM reciever )

    The use of these 4 lines is completely optional and elective... in the NorthUp compass, these 4 pins run directly to 4 uncommitted wire pads, provided to allow a hardwire connection to the ( nearby ) antenna circuits.


    Additional Information


    Additional information ( if desired ) can be obtained by e-mail from the development engineer :

    Bob Simmons